Transfer coefficients for evaporation

Abstract

An analysis of the recent experimental results by Fang and Ward is given. They found a temperature jump almost upto 10 K across the evaporating surface of water, octane and methylcyclohexane. We use nonequilibrium thermodynamics to obtain the appropriate boundary conditions at the surface. Interfacial transfer coefficients, appearing in these boundary conditions, can then be determined from the experiments. We present them in a form useful for engineering calculations. A comparison is made with the predictions for the transfer coefficients from kinetic theory. For the three materials the kinetic theory values were found to be 30 to 100 times larger than those found from the experiment. It is explained why this gives a liquid surface, which is colder than the adjacent vapor, contrary to the prediction by the kinetic theory. The relative magnitude that we find for the interfacial transfer coefficients, suggests that the condensation coefficient of the kinetic theory decreases with increasing internal degrees of freedom in a molecule. However, a lower value of this coefficient is not sufficient to explain the small value of the transfer coefficients. As a possible explanation for this, we forward the hypothesis that the single-particle collision model used in the kinetic theory for this phenomenon, should be modified to account for multiparticle events.